Defensive strategies in Geranium sylvaticum, Part 2: Roles of water-soluble tannins, flavonoids and phenolic acids against natural enemies

Hydrolyzable tannins are incorporated into the endocarp during sclerification of the water caltrop Trapa natans

The water caltrop (Trapa natans) develops unique woody fruits with unusually large seeds among aquatic plants. During fruit development, the inner fruit wall (endocarp) sclerifies and forms a protective layer for the seed. Endocarp sclerification also occurs in many land plants with large seeds; however, in T. natans, the processes of fruit formation, endocarp hardening, and seed storage take place entirely underwater. To identify potential chemical and structural adaptations for the aquatic environment, we investigated the cell-wall composition in the endocarp at a young developmental stage, as well as at fruit maturity. Our work shows that hydrolyzable tannins-specifically gallotannins-flood the endocarp tissue during secondary wall formation and are integrated into cell walls along with lignin during maturation. Within the secondary walls of mature tissue, we identified unusually strong spectroscopic features of ester linkages, suggesting that the gallotannins and their derivatives are cross-linked to other wall components via ester bonds, leading to unique cell-wall properties. The synthesis of large amounts of water-soluble, defensive aromatic metabolites during secondary wall formation might be a fast way to defend seeds within the insufficiently lignified endocarp of T. natans.

Mass Spectrometric Fingerprint Mapping Reveals Species-Specific Differences in Plant Polyphenols and Related Bioactivities

Plant species show large variation in the composition and content of their tannins and other polyphenols. These large metabolites are not easy to measure accurately, but they are important factors for species bioactivity and chemotaxonomy. Here, we used an automated group-specific UHPLC-DAD-MS/MS tool to detect and quantify eight most common polyphenol groups in 31 chemically diverse plant species representing many types of growth forms and evolutionary ages. Ten replicate plants were used for each species and two polyphenol-related bioactivities, i.e., protein precipitation capacity and oxidative activity were measured in all samples as well. By the help of a novel 2D fingerprint mapping tool we were able to visualize the qualitative and quantitative differences between the species in hydrolysable tannins (galloyl and hexahydroxydiphenoyl derivatives), proanthocyanidins (procyanidins and prodelphinidins), flavonols (kaempferol, quercetin and myricetin derivatives) and quinic acid derivatives together with the two bioactivities. The highest oxidative activities were found with species containing ellagitannins (e.g., Quercus robur, Geranium sylvaticum, Lythrum salicaria and Chamaenerion angustifolium) or prodelphinidin-rich proanthocyanidins (e.g., Ribes alpinum, Salix phylicifolia and Lysimachia vulgaris). The best species with high protein precipitation capacity were rich in gallotannins (Acer platanoides and Paeonia lactiflora) or oligomeric ellagitannins (e.g., Comarum palustre, Lythrum salicaria and Chamaenerion angustifolium). These types of tools could prove their use in many types of screening experiments and might reveal even unusually active polyphenol types directly from the crude plant extracts.

From soil to plant: strengthening carrot defenses against Meloidogyne incognita with vermicompost and arbuscular mycorrhizal fungi biofertilizers

Introduction

Sustainable agricultural practices for controlling crop pests are urgently needed to reduce the reliance on chemical pesticides, which have long-term detrimental effects on ecosystems. In this study, we assessed the effectiveness of arbuscular mycorrhizal fungi (AMF) and vermicompost (Vc) supplementation, alone and in combination, in mitigating the negative impacts of Meloidogyne incognita infestation on carrot (Daucus carota L.) growth, development, and physiology.

Methods

We measured different plant growth parameters such as plant height and biomass accumulation, several plant physiological parameters such as the levels of photosynthetic pigments, phenolics, and the activity of defense enzymes such as peroxidases and polyphenol oxidases, and evaluated the severity of Meloidogyne incognita nematode infestation on plants treated or not treated with vermicompost (Vc) and/or arbuscular mycorrhizal fungi (AMF).

Results

Our findings show that M. incognita significantly affects plant growth, biomass accumulation, and photosynthetic pigment and carotenoid content. The incorporation of Vc and AMF into the soil, either individually or in combination, significantly alleviates the negative effects of nematode infestation on carrot plants. This was accompanied by the induction of phenolic compounds and defense enzymes such as peroxidases (+15.65%) and polyphenol oxidases (29.78%), and by a reduction in the severity of nematode infestation on Vc and AMF-treated plants compared to nematode-infested plants. Principal component analysis (PCA) shows significant correlations between various of the studied parameters. In particular, we observed negative correlations between the application of AMF and Vc alone and in combination and disease severity, and positive correlations between plant growth, photosynthetic pigments phenol content, and activity of defense enzymes.

Discussion

Our study highlights the relevance of cultural practices and beneficial microorganisms for the sustainable and environmentally friendly management of agricultural pests.

Pollen-feeding behavior of diverse insects on Geranium delavayi, a flower with large, accessible pollen grains

PREMISE

Why have pollen grains evolved to be exceptionally large in some species? Pollen-feeding hypothesis suggests that if the proportion of pollen amounts for feeding is reduced in a flower, the low allocation to pollen number would allow pollen grains to be larger.

METHODS

To examine whether species with large pollen grains experience low pollen consumption, the behavior of insects feeding on nectar and pollen was observed and pollen transfer efficiency was estimated for four visitor types in Geranium delavayi. To see whether bees actively collected pollen, the numbers of grains in pollen baskets and on the body were compared. Both nutritional value (total protein and lipid) and chemical defense (phenolic metabolites) in pollen against pollen feeders were measured.

RESULTS

Bumblebees and honeybees foraged for nectar, rarely groomed pollen into corbiculae, and had >5× higher pollen transfer efficiency than smaller solitary bees and flies, which were pollen eaters that removed more pollen but deposited less. Pollen grains were characterized by low protein and high lipid content with a low protein-lipid ratio, an unfavorable combination for bumblebees. Three secondary metabolites were significantly higher in pollen grains (7.77 mg/g) than in petals (1.08 mg/g) or in nectar (0.44 mg/g), suggesting stronger chemical defense in pollen.

CONCLUSIONS

Our results indicated that large bees took nectar but little of the nutritionally poor and highly toxic pollen. These data support one prediction of the pollen-feeding hypothesis, that species with few and large pollen grains would also have low pollen-consumption rates.

Metabolic and physiological alterations indicate that the tropical broadleaf tree Eugenia uniflora L. is sensitive to ozone

Eugenia uniflora L. is an important fruit tree native to tropical South America that adapts to different habitats, thanks to its metabolic diversity and ability to adjust the leaf antioxidant metabolism. We hypothesized that this metabolic diversity would also enable E. uniflora to avoid oxidative damage and tolerate the enhanced ozone (O3) concentrations that have been registered in the (sub)tropics. We investigated whether carbohydrates, polyphenols and antioxidants are altered and markers of oxidative damage (ROS accumulation, alterations in leaf gas exchange, growth and biomass production) are detected in plants exposed to two levels of O₃ (ambient air and twice elevated ozone level in a O₃-FACE system for 75 days). Phytotoxic O₃ dose above a threshold of 0 nmol m^-2 s^-1 (POD0) and accumulated exposure above 40 ppb (AOT40) were 3.6 mmol m^-2 and 14.898 ppb h at ambient, and 4.7 mmol m^-2 and 43.881 ppb h at elevated O₃. Twenty-seven primary metabolites and 16 phenolic compounds were detected in the leaves. Contrary to the proposed hypothesis that tropical broadleaf trees are relatively O₃ tolerant, we concluded that E. uniflora plants are sensitive to elevated O₃ concentrations. Experimental POD0 values were lower than the critical levels for visible foliar O₃, because of low stomatal conductance. In spite of this low stomatal O₃ uptake, we found classic O₃ injury, e.g. reduction in carbohydrates and fatty acids concentrations; non-significant changes in the polyphenol profile; inefficient antioxidant responses; increased contents of ROS and indicators of lipid peroxidation; reductions in stomatal conductance, net photosynthesis, root/shoot ratio and height growth. However, we also found some compensation mechanisms, e.g. increased leaf concentration of polyols for protecting the membranes, and increased leaf number for compensating the decline of photosynthetic rate. These results help filling the knowledge gap about tropical tree responses to O₃.

DNA Related Enzymes as Molecular Targets for Antiviral and Antitumoral Chemotherapy. A Natural Overview of the Current Perspectives

BACKGROUND

The discovery of new chemotherapeutic agents still remains a continuous goal to achieve. DNA polymerases and topoisomerases act in nucleic acids metabolism modulating different processes like replication, mitosis, damage repair, DNA topology and transcription. It has been widely documented that Polymerases serve as molecular targets for antiviral and antitumoral chemotherapy. Furthermore, telomerase is a ribonucleoprotein with exacerbated activity in most of the tumor cell lines, becoming as an emergent target in Cancer treatment.

METHODS

We undertook an exhaustive search of bibliographic databases for peer-reviewed research literature related to the last decade. The characteristics of screened bibliography describe structure activity relationships and show the principal moieties involved. This work tries to summarize the investigation about natural and semi-synthetic products with natural origin with the faculty to inhibit key enzymes that play a crucial role in DNA metabolism.

RESULTS

Eighty-five data references were included in this review, showing natural products widely distributed throughout the plant kingdom and their bioactive properties such as tumor growing inhibitory effects, and anti-AIDS activity.

CONCLUSION

The findings of this review confirm the importance to find new drugs and biologically active natural products, and their potential medicinally useful benefits.

Proliferation study and microscopy evaluation on the effects of tannic acid in human fetal osteoblast cell line (hFOB 1.19)

Tannic acid (TA) is a phenolic compound that might act directly on osteoblast metabolism. The study was performed to investigate the effects of TA on the proliferation, mineralization, and morphology of human fetal osteoblast cells (hFOB 1.19). The cells were divided into TA-treated, untreated, and pamidronate-treated (control drug) groups. Half maximal effective concentration (EC₅₀ ) values for TA and pamidronate were measured using MTT assay. The EC₅₀ of hFOB 1.19 cells treated with TA was 2.94 M. This concentration was more effective compared to the pamidronate (15.27 M). Cell proliferation assay was performed to compare cell viability from Day 1 until Day 14. The morphology of hFOB 1.19 was observed via inverted microscope and scanning electron microscope. Calcium (Ca) and phosphate (P) were assessed using energy-dispersive X-ray (EDX) analysis. Furthermore, the mineralization of hFOB 1.19 was determined by von Kossa staining (P depositions) and Alizarin Red S staining (Ca depositions). The number of cells treated with TA was significantly higher than the two control groups at Day 10 and Day 14. The morphology of cells treated with TA was uniformly fusiform-shaped with filopodia extensions. Besides, globular-like structures of deposited minerals were observed in the TA-treated group. In line with other findings, EDX spectrum analysis confirmed the presence of Ca and P. The cells treated with TA had significantly higher percentage of both minerals at Day 3 and Day 10 compared to the two control groups. In conclusion, TA enhances cell proliferation and causes cell morphology changes, as well as improved mineralization.

Opinion on the Hurdles and Potential Health Benefits in Value-Added Use of Plant Food Processing By-Products as Sources of Phenolic Compounds

Plant foods, their products and processing by-products are well recognized as important sources of phenolic compounds. Recent studies in this field have demonstrated that food processing by-products are often richer sources of bioactive compounds as compared with their original feedstock. However, their final application as a source of nutraceuticals and bioactives requires addressing certain hurdles and challenges. This review discusses recent knowledge advances in the use of plant food processing by-products as sources of phenolic compounds with special attention to the role of genetics on the distribution and biosynthesis of plant phenolics, as well as their profiling and screening, potential health benefits, and safety issues. The potentialities in health improvement from food phenolics in animal models and in humans is well substantiated, however, considering the emerging market of plant food by-products as potential sources of phenolic bioactives, more research in humans is deemed necessary.

Fractionation of the more active extracts of Geranium molle L.: a relationship between their phenolic profile and biological activity

Geranium molle L., commonly known as Dove's-foot Crane's-bill or Dovesfoot Geranium, is an herbaceous plant belonging to the Geraniaceae family. Contrary to many other Geranium species, the bioactivity and the phytochemical composition of G. molle seem not to have attracted attention until a recent study from our group regarding the bioactivity of several aqueous and organic extracts of the plant. In particular, we assessed the cytotoxic activity of these extracts against several human tumor cell lines (breast, lung, cervical and hepatocellular carcinomas) and a non-tumor porcine liver primary cell line, inspired by an ethnopharmacological report describing the traditional use of this medicinal plant in some regions of Northeast Portugal for the treatment of cancer. Following this preliminary evaluation, the most active extracts (acetone and methanol) were fractionated by column chromatography and the resulting fractions were evaluated for their antioxidant activity and cytotoxicity against the same cell lines. The bio-guided fractionation of the extracts resulted in several fractions exhibiting improved bioactivity in comparison with the corresponding crude extracts. The fractions obtained from the acetone extract consistently displayed the lowest EC50 and GI50 values and presented the highest content of total phenolic compounds. The phytochemical composition of the most bioactive fractions of the acetone and methanol extracts was also determined and about thirty compounds, mainly flavonoids and phenolic acids, could be identified for the first time in G. molle.

Effect of Polyphenols From Campomanesia adamantium on Platelet Aggregation and Inhibition of Cyclooxygenases: Molecular Docking and in Vitro Analysis

Campomanesia adamantium is a medicinal plant of the Brazilian Cerrado. Different parts of its fruits are used in popular medicine to treat gastrointestinal disorders, rheumatism, urinary tract infections and inflammations. Despite its widespread use by the local population, the mechanisms involving platelet aggregation and the inhibition of cyclooxygenase by C. adamantium are unknown. This study evaluated the chemical composition, antioxidant activities and potential benefits of the C. adamantium peel extract (CAPE) and its components in the platelet aggregation induced by arachidonic acid in platelet-rich plasma. Aspects of the pharmacological mechanism were investigated as follows: platelet viability, calcium mobilization, levels of the cyclic nucleotides cAMP and cGMP, thromboxane B₂ levels, and the inhibitory effects on COX-1 and COX-2 were studied in vitro and using molecular docking in the catalytic domain of these proteins. The major CAPE constituents standing out from the chemical analysis are the flavonoids, namely those of the flavones and chalcones class. The results showed that CAPE, quercetin and myricetin significantly decreased arachidonic acid-induced platelet aggregation; the assays showed that CAPE and quercetin decreased the mobilization of calcium and thromboxane B₂ levels in platelets and increased cAMP and cGMP levels. Moreover, CAPE inhibited the activity of COX-1 and COX-2, highlighting that quercetin could potentially prevent the access of arachidonic acid more to the catalytic site of COX-1 than COX-2. These results highlight CAPE’s potential as a promising therapeutic candidate for the prevention and treatment of diseases associated with platelet aggregation.

Variability between organs of proanthocyanidins in Geranium sylvaticum analyzed by off-line 2-dimensional HPLC-MS

Proanthocyanidins have been characterized in the different organs of Geranium sylvaticum by using an off-line 2-dimensional chromatographic method. In the first dimension, proanthocyanidins were separated according to their increasing degree of polymerization by hydrophilic interaction liquid chromatography. In the second dimension, reversed-phase liquid chromatography was used to separate the same size isomers based on their hydrophobicity. Ultraviolet detection and high-resolution electrospray ionization time-of-flight mass spectrometry were used for the identification and characterization of proanthocyanidins. The method was proven to be suitable for the analysis of plant organ fractions of G. sylvaticum that contained complex mixtures of phenolic compounds and proanthocyanidins from several classes. Procyanidins, prodelphinidins and mixed proanthocyanidins composed of both (epi)catechin and (epi)gallocatechin units including monomers, oligomers and polymers up to the degree of polymerization of 20 were identified. The type and size of proanthocyanidins varied between eight studied organs of G. sylvaticum. The 2-dimensional analysis revealed that each proanthocyanidin type occur in the equally complex isomeric pattern. The mean degree of polymerization of different proanthocyanidins in the eight plant organs of G. sylvaticum was 5 and it varied only slightly between plant organs.

Constitutive phenolic biomarkers identify naïve Quercus agrifolia resistant to Phytophthora ramorum, the causal agent of sudden oak death

Sudden oak death, caused by the invasive pathogen Phytophthora ramorum Werres, de Cock & Man in't Veld, can be deadly for Quercus agrifolia Neé (coast live oak, CLO). However, resistant trees have been observed in natural populations. The objective of this study was to examine if pre-attack (constitutive) levels of phenolic compounds can be used as biomarkers to identify trees likely to be resistant. Naïve trees were selected from a natural population and phloem was sampled for analysis of constitutive phenolics. Following P. ramorum inoculation, trees were phenotyped to determine disease susceptibility and constitutive phenolic biomarkers of resistance were identified. Seasonal variation in phloem phenolics was also assessed in a subset of non-inoculated trees. Four biomarkers, including myricitrin and three incompletely characterized flavonoids, together correctly classified 80% of trees. Biomarker levels were then used to predict survival of inoculated CLO and the proportion of resistant trees within a subset of non-inoculated trees from the same population. Levels of five phenolics were significantly affected by season, but with no pronounced variation in average levels among seasons. These results suggest that pre-infection levels of specific phenolic compounds (i.e., biomarkers) can identify trees naturally resistant to this invasive forest pathogen. Knowledge of resistant trees within natural populations may be useful for conserving and breeding resistant trees and for disease management.

Hydrolyzable Tannins, Flavonol Glycosides, and Phenolic Acids Show Seasonal and Ontogenic Variation in Geranium sylvaticum

The seasonal variation of polyphenols in the aboveground organs and roots of Geranium sylvaticum in four populations was studied using UPLC-DAD-ESI-QqQ-MS/MS. The content of the main compound, geraniin, was highest (16% of dry weight) in the basal leaves after the flowering period but stayed rather constant throughout the growing season. Compound-specific mass spectrometric methods revealed the different seasonal patterns in minor polyphenols. Maximum contents of galloylglucoses and flavonol glycosides were detected in the small leaves in May, whereas the contents of further modified ellagitannins, such as ascorgeraniin and chebulagic acid, increased during the growing season. In flower organs, the polyphenol contents differed significantly between ontogenic phases so that maximum amounts were typically found in the bud phase, except in pistils the amount of gallotannins increased significantly in the fruit phase. These results can be used in evaluating the role of polyphenols in plant-herbivore interactions or in planning the best collection times of G. sylvaticum for compound isolation purposes.

Chemical characterization and bioactive properties of aqueous and organic extracts of Geranium robertianum L

Geranium robertianum L. has been used in folk medicine and herbalism practice for the treatment of various conditions, but the study of its bioactivity has been barely addressed. Although its phytochemical composition has received some attention, contributions to the nutritional composition are practically unknown. Herein, G. robertianum gathered in Trás-os-Montes, Northeastern Portugal, was chemically characterized regarding nutritional parameters, and the antioxidant activity and cytotoxicity against several human tumor cell lines and non-tumor porcine liver primary cells of several aqueous and organic extracts were evaluated. G. robertianum showed to be an equilibrated valuable herb, rich in carbohydrates and proteins, and poor in fat, providing sugars, tocopherols, organic and essential fatty acids. Amongst the extracts, the acetone one showed the highest total phenol and total flavonoid contents, as well as the greatest antioxidant and cytotoxic activities. This extract showed to contain hydrolysable tannins (e.g. geraniin and castalagin/vescalagin), as the main phenolic compounds.

Chemical characterization and bioactive properties of Geranium molle L.: from the plant to the most active extract and its phytochemicals

After a period of indifference, in which synthetic compounds were favored, there is an increasing interest in the study of the biological properties of plants and the active principles responsible for their therapeutic properties. Geranium molle L. has been used in the Portuguese folk medicine for the treatment of various ailments including cancer but, unlike many of the species from the Geranium genus, its phytochemical characterization and biological activity are virtually unexplored. In this study a G. molle sample from Trás-os-Montes, north-eastern Portugal, was chemically characterized regarding nutritional value, free sugars, organic acids, fatty acids and tocopherols, and several aqueous (decoction, infusion) and organic (n-hexane, dichloromethane, ethyl acetate, acetone, methanol) extracts of the plant were assessed for their bioactive properties. The antioxidant activity was evaluated by means of the free radicals scavenging activity, reducing power and inhibition of lipid peroxidation. The cytotoxicity of the different extracts was assessed in vitro against several human cancer cell lines (breast, lung, cervical and hepatocellular carcinomas) and, additionally, their hepatotoxicity was evaluated using a porcine liver primary cell culture. G. molle was shown to be rich in carbohydrates and proteins, providing tocopherols and essential fatty acids. Amongst the various extracts, the acetone extract was found to have the highest content of phenolic compounds (mainly ellagitannins, but also some flavone and flavonol glycosides) as well as the highest antioxidant and cytotoxic activities. To the best of our knowledge, this is the first report on the chemical composition and bioactive properties of G. molle.

Tea polyphenols ameliorate fat storage induced by high-fat diet in Drosophila melanogaster

Background

Polyphenols in tea are considered beneficial to human health. However, many such claims of their bioactivity still require in vitro and in vivo evidence.

Results

Using Drosophila melanogaster as a model multicellular organism, we assess the fat accumulation-suppressing effects of theaflavin (TF), a tea polyphenol; epitheaflagallin (ETG), which has an unknown function; and epigallocatechin gallate (EGCg), a prominent component of green tea. Dietary TF reduced the malondialdehyde accumulation related to a high-fat diet in adult flies. Other physiological and genetic responses induced by the high-fat diet, such as lipid accumulation in the fat body and expression of lipid metabolism-related genes, were ameliorated by the addition of TF, ETG, and EGCg, in some cases approaching respective levels without high-fat diet exposure. Continuous ingestion of the three polyphenols resulted in a shortened lifespan.

Conclusion

We provide evidence in Drosophila that tea polyphenols have a fat accumulation-suppressing effect that has received recent attention. We also suggest that tea polyphenols can provide different desirable biological activities depending on their composition and the presence or absence of other chemical components.

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Tea polyphenols have a fat accumulation-suppressing effect in Drosophila.

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Dietary theaflavin ameliorates high-fat diet-induced hydroperoxidase accumulation.

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The novel tea polyphenol epitheaflagallin strongly suppresses lipid accumulation.

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The beneficial effects of tea polyphenols can be enhanced by altering composition.

Sylvatiins, acetylglucosylated hydrolysable tannins from the petals of Geranium sylvaticum show co-pigment effect

Four hydrolysable tannins, named as sylvatiins A (1), B (2), C (3) and D (4), were isolated from the petals of Geranium sylvaticum. On the basis of spectrometric evidence of NMR analysis ((1)H NMR, (13)C NMR, DQF-COSY, TOCSY, NOESY, HSQC and HMBC), circular dichroism (CD) and ESI-MS/MS, sylvatiins A, B and C were characterized as galloyl glucoses containing one or two acetylglucoses attached to the 3-OH of the galloyl group, whereas sylvatiin D was found to have a chebulinic acid core containing acetylglucose attached in a similar way. The potential of these compounds to act as defensive compounds against herbivores was evaluated using the radial diffusion assay that measures the protein precipitation capacity. In addition, the capacity of sylvatiins to act as co-pigments with anthocyanins of G. sylvaticum petals was measured in vitro at different pH values. Sylvatiins A and D maintained efficiently the purple flower color near the natural pH of petal cells. The amount of sylvatiins was changed according to the flower color; deep purple petals with higher amount of anthocyanin contained more sylvatiins A and C than whiter petals. It was concluded that G. sylvaticum petal cells may accumulate sylvatiins for intermolecular co-pigmentation purposes.

Rapid estimation of the oxidative activities of individual phenolics in crude plant extracts

Previous studies of purified phenolic compounds have revealed that some phenolics, especially ellagitannins, can autoxidise under alkaline conditions, which predominate in the midgut of lepidopteran larvae. To facilitate screening for the pro-oxidant activities of all types of phenolic compounds from crude plant extracts, we developed a method that combined our recent spectrophotometric bioactivity method with an additional chromatographic step via UPLC-DAD-MS. This method allowed us to estimate the total pro-oxidant capacities of crude extracts from 12 plant species and to identify the individual phenolic compounds that were responsible for the detected activities. It was found that the pro-oxidant capacities of the plant species (i.e., the concentrations of the easily-oxidised phenolics) varied from 0 to 57 mg/g dry wt, representing from 0% to 46% of the total phenolics from different species. UPLC-DAD-MS analysis revealed that most flavonol and flavone glycosides were only slightly affected by alkaline conditions, thus indicating their low pro-oxidant activity. Interestingly, myricetin-type compounds differed from the other flavonoids, as their concentrations decreased strongly due to alkaline incubation. The same effect was detected for hydrolysable tannins and prodelphinidins, suggesting that a pyrogallol sub-structure could be a key structural component that partially explains their easy oxidation at high pH. Other types of phenolic compounds, such as hydroxycinnamic acids, were relatively active, as well. These findings demonstrate that this method displays the potential to identify most of the active and inactive pro-oxidant phenolic compounds in various plant species.